As is well known, many apparatus for recording information on a recording medium, such as an optical disc, have been developed.
In the recording information apparatus, a light beam is projected to an optical disc for recording or reproducing information to or from the optical disc by an optical head. The optical head is moved in the radial direction of the optical disc, while the optical disc, rotates.
The recording information apparatus includes an objective lens for projecting the light beam onto the optical disc. Thus, the light beam is converged to the optical disc so that a spot of the light beam is formed on the surface of the optical disc. The recording information apparatus further includes a recording information apparatus for controlling the spot so that it follows a predetermined track on the optical disc. Such a tracking control of the spot is carried out by controlling positions of the objective lens with respect to the optical head and the optical head to with respect the optical disc in the radial direction of the optical disc using a tracking error signal detected from the light beam reflected from the optical disc. That is, a servo control system is established in the tracking control operation.
In the tracking control operation, the light beam reflected from the optical disc is applied to at least a pair of light detector cells provided in the optical head again passing through the objective lens. The light detector cells are separated from each other in the radial direction of the optical disc. Detected signals of the two light detector cells are subtracted from each other in a tracking error detector such as a differential amplifier. If the spot is offset from the center of the track, a relatively large error signal of the detected signals, i.e., a tracking error signal, is output from the tracking error detector. Such an offset of the spot from the predetermined track occurs due to dust on the optical disc or a mechanical shock applied to the recording information apparatus by an external force. That is, the objective lens and/or the optical head is shifted in the radial direction of the optical disc from its regulated position. The tracking error signal is applied to the recording information apparatus. The recording information apparatus moves either or both the objective lens and the optical head to reduce the offset of the spot from the predetermined track. Accordingly, the tracking of the spot to the predetermined track is secured.
In a conventional recording information apparatus, however, the tracking control operation is not sufficiently carried out in a case when the objective lens and/or the optical head has been rapidly shifted, because the servo control system of the tracking control operation diverges in the case of the rapid shift of the objective lens and/or the optical head.